Radio receiving system



Sept. 7, 1937. H. MAEDA RADIO RECEIVING SYSTEM SBQ Bubm,

Filed June 4, 1935 afb,

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Patented Sept. 7 1937 UNITED STATES vPATENT OFFICE RADIO RECEIVING SYSTEM Application June 4, 1935, Serial No. 24,839 In Japan August 13, k1934 4 Claims.

This invention relates to radio receiving systems and more particularly to detecting systems therefor, in which thermionic tubes are operated with A. C. anode current of approximately saturation value.

An object of this invention is to obtain very sensitive detection of received waves with a tube which is so operated that its A. C. anode current varies according to drooping characteristics.

Another object of this invention is to provide an improved autodyne system for obtaining detection on the heterodyne principle with a single tube operating as a detector and as a local oscillator, in which a more sensitive detection is obtained than in the ordinary autodyne system.

A further object of this invention is to provide a single-tube zero-beat detecting system having a high sensitivity.

A still further object of this invention is to provide a power amplifying and detecting system having a high sensitivity.

There are other objects and particularities of this invention, which together with the nature and construction of this invention, will appear in the following description with reference to the accompanying drawing, wherein Fig. 1 shows diagrammatically` an ordinary autodyne radio receiving system. V

Fig. 2 is a diagram showing the dynamic characteristic curves of the system shown in Fig. 1.

Fig. 3 shows diagrammatically one embodiment of this invention.

Fig. 4 is a diagram showing the dynamic characteristic curves of the system shown in Fig. 3.

Fig. 5 shows a modification of the system shown in Fig. 3.

Fig. 6 shows diagrammatically another embodiment of thisinvention which enables power amplifying detection with a high sensitivity.

This invention will best be understood with reference to the ordinary autodyne system shown in Fig. l. The system comprises a resonance circuit I which istuned exactly to local oscillation frequency f2 and approximately to incoming oscillation frequency f1 from an antenna 2 and connected to the grid and cathode of an ordinary three-electrode thermionic tube 3. The tube 3 has its anode circuit in feed-back coupling with the resonance circuit I through a coupling coil 4, and the local oscillation is superposed on the incoming oscillation in the circuit I. The frequencies f1 and f2 are so selected that there is created a beat having a frequency (f1-f2) or (fw-f1), This beat is detected or rectiiied by the tube 3 and transmitted to the receiver 5 through (Cl. Z50-20) an intermediatefrequency amplifier and a second detector as usual. The coupling coils B between the anode circuit of the tube 3 and the succeeding stage i. e. the intermediate-frequency amplier is provided with a by-pass condenser 'I 5 for the local oscillation f2. This condenser provides a high impedance to the beat frequency and the latter passes through the primary coil 6.

It is well known that such a system of heterodyne principle as shown in Fig. 1 has very low sensitivity because of the fact that it operates near the saturation point of the dynamic characteristic curve, if the oscillation stiffness is made so high that the local oscillation is maintained steady.

Referring to Fig. 2, curve Ip shows the anode current, while curve Ig shows the grid current, with respect to the grid potential Eg of the tube 3. As is well-known, the drooping characteristic shown of the anode current curve Ip is due to the high potential drop across the coil 4 which is coupled to the resonance circuit I and consequently provides a very high impedance to the local frequency f2. In order to maintain the local oscillation f2 generated by the tube 3 steady, the 25 oscillation stiffness must be so high thatthe tube operates near the saturation point A of the anode current curve Ip. Under the circumstances, the beat (f1-f2) or (f2-f1) exciting the grid of the tube 3 cannot sensibly be detected. This is true even with multi-grid tubes when detected output is to be obtained from the anode circuit.

According to this invention, a therrnionic tube having an additional grid is used in place of the tube 3, and the output circuit is connected to this additional grid. Referring now to Fig. 3, a resonance circuit II is coupled with the antenna I2 and is tuned exactly to the local frequency f2 and approximately to the incoming frequency f1, the local oscillation f2 being fed by a thermionic tube I3 to the circuit I I through a coil I4 inductively coupled therewith. The tube I3 comprises a cathode 2|, an anode 22, a control grid 23 and a second grid 24 disposed between the anode and the control grid. The second grid may, for example, be the screen grid of RCA- 24-A tube or RCA--B'l tube, or the acceleration grid of RCA-47 tube.

The local oscillation feed-back circuit is formed between the anode 22 and the cathode 2I through the coil I4 and a stopping condenser 25 for preventing direct current flowing therethrough, and the anode 22 is energized to a positive potential by a battery 26 through a choke Cit coil 21 for preventing the oscillation current flowing therethrough. The screen grid 24 is connected to one end of a coupling transformer I6, the other end thereof being connected to the cathode 2| through the battery 26. A condenser Il provides a low impedance by-pass around the transformer I5 for the frequency f2. The direct current resistances offered by coils I4, 2'I, I6 and 56 are negligible. The anode should be maintained at a high direct voltage not materially lower than that of the second grid. It is not always intended to maintain the second grid at a higher direct current than the anode, but the former may be substantially equal to the latter.

With the above arrangement, the anode current Ip still has a drooping characteristic as shown in Fig. 4 due to the high impedance of the coil I4 coupled to the resonance circuit I I, but the screen grid current Isg has a sharp rising characteristic, especially from the approximate point B corresponding to the saturation point A of the anode current curve. According to this invention, this screen grid current is utilized for detecting purpose near the point B, and avery sensitive detection is secured. The output of the beat or intermediate frequency is transmitted to succeeding amplifier 28 and detected by second detector 29 which in turn Supply low frequency waves to amplifier 3U, and the signal is received by a receiver I5.

Although signal Eg without modulation is illustrated in Figs. 2 and 4, it is obvious that the above description is true also when the signal has voice modulation.

In the ordinary autodyne system, it must also be noted that the control grid current Ig becomes very high as shown in Fig. 2 due to the fact that when the grid potential Eg is near the positive maximum, the anode potential becomes less positive or more negative. This increase in grid current is undesirable because it increases the damping coeiicient of the tuned circuit. Contrary to this, according to this invention, the control grid current Ig is kept sufliciently low to maintain the damping effect very low.

With the arrangement shown in Fig. 3, when the local frequency is synchronized with the incoming frequency, that is to say f1=f2, it is obvious that a zero-beat detection is effected with a very high sensitivity.

A portion of the arrangement shown in Fig. 3 may be modified as shown in Fig. 5. In this modication, the local oscillation circuit and incoming frequency circuit are separated from each other, the anode load circuit for the local oscillation being formed between the anode 22 and the cathode 2| by a condenser 4I and an inductance coil 42 which is inductively coupled to a coil 43 in the grid return circuit. The operation of this modified arrangement is quite similar to that shown in Fig. 3.

Generally stated, the operation of an oscillator tube is equivalent to that of an amplifiertube with a high grid excitation voltage, and the feature of this invention embodied in the arrangement shown in Fig. 3, is also applicable to power amplifying detectors in which the grid excitation voltage is made very high.

Now referring to Fig. 6, an antenna 5I is supplied with energy from a local oscillation generator 52. This energy is superposed on the incoming energy, and a very high potential is applied to the control grid 23 of the tube I3' which is similar to the tube I3 shown in Figs. 3 and 5. The output circuit for the amplified oscillation is formed between the anode 22 and the cathode 2|' by a condenser 53 and an inductance coil 54 through a D. C. stopping condenser 55, while the D. C. circuit is formed between the same electrodes through a choke coil 56 and a high potential battery 51. According to this invention, the screen grid 24 is used for detected-waves output circuit and is connected to the positive terminal of the battery 5l through a coupling transformer I6 provided with a by-pass condenser I'I for high frequency waves, and the detected waves in the screen grid circuit is transmitted to the succeeding stage, not shown, by the transformer I6'.

As will be obvious from an inspection of the several forms of this invention as exemplified in the drawing, a feature common to all of such forms is resident in the fact that a thermionic tube having a cathode, an anode, a control grid and a second grid disposed between the anode and the control grid is used, and the second grid is utilized to obtain the output. By such arrangement, a very sensitive detection is effected in a` radio receiving system, in which a thermionic tube is operated near the saturation point of its anode current curve and the detecting sensitivity is, otherwise, consequently very low.

I claim as my invention:-

1. In a detecting circuit for radio receiving system, the combination with a thermionic tube having an anode, a cathode, a control grid and a second grid disposed between said anode and said control grid and to be operated with A. C. anode current of approximately saturation value, of means for generating high frequency oscillation including said anode and said control grid, and means for obtaining output including said second grid, means for applying a unidirectional potential to the electrodes and for maintaining said anode to a high direct voltage not materially lower than that of said second grid.

2. A detecting circuit for radio receiving system, comprising means for generating a local oscillation, a resonance circuit substantially tuned to incoming frequency and to the frequency of said local oscillation, a thermionic tube having an anode, a cathode, a control grid and a second grid disposed between said anode and said control grid, said control grid being excited from said resonance circuit, an anode load circuit for said local oscillation connected to said anode, an output circuit connected to said second grid, means to apply a unidirectional potential to the electrodes and to maintain said second grid to a high direct voltage with respect to said cathode, and said anode to a high direct voltage not materially lower than that of said second grid.

3. A radio detecting system comprising a resonance circuit substantially tuned to incoming frequency, a thermionic tube having an anode, a cathode, a control grid and a second grid disposed between said anode and said control grid, said control grid being excited from said resonance circuit, a feed back circuit connected to said anode for producing a local oscillation, means for coupling together said resonance circuit and said feed back circuit to produce a beat frequency, an output circuit for said beat frequency including said second grid, and a high direct voltage source with its positive terminal connected to said second grid and to said anode with its negative terminal connected to said cathode, said output circuit further including a coupling transformer with a high frequency by-pass condenser.

4. A radio receiving system, comprising an antenna, an input circuit receiving energy from and a second grid, said mst-mentioned grid being excited by high alternating current potential from said input circuit, means for applying a high positive direct voltage to said second grid, means for applying a unidirectional potential to the electrodes and for maintaining said anode to a high positive direct voltage not materially lower than that of said second grid, an anode load circuit for the local oscillation formed between said anode and said cathode, and an output circuit formed between said second grid and said cathode.

HISAO MAEDA. 

